The present invention relates to a toner used for electrophotographic reproduction.
In electrophotographic reproduction, the electrostatic latent images may be developed by a developer to visualize. Nowadays, among processes for development, magnetic brush development is mainly employed. The developers used in this method include those of two-component system using toner and carrier containing magnetic powder or those of single-component system wherein toner contains magnetic powder. In magnetic brush method, toner is charged by, for example, triboelectrification and introduced into development zone utilizing magnetic force which the magnetic powder is received, and after development of the electrostatic latent images, transfer and fixing of toner, duplicated images are formed.
Dustability is one of the basic properties to estimate toner. A toner having great dustability is accompanied with several problems such as generation of fog. defective picture quality, unstable density of image, contamination of the inside of a copying machine with toner.
The dusting of toner may occur because the toner has widely distributing charge and the low-charged components thereof, which are not constrained by electrostatic attractive force of carrier on a sleeve, may scatter.
Recently, as a toner for electrostatic development, high definition in line-duplicability, high picture quality in texture, dot duplicability, gradation, resolving power and the like have been required. In order to attain such objects, toner particles are required to be minimized. Such minimization of toner particles, however, provides unfavorable side effect, for example, deterioration of electrification owing to reduction of surface area of toner, deterioration of fluidity of developer or toner itself and the like, and remarkable increase of dusting.
To avoid dusting of toner, a method is proposed wherein magnetic field formed by a magnet roller placed between sleeve and photosensitive member is utilized to constrain toner by its magnetic force. For this purpose, toner containing inorganic magnetic powder is effectively used as toner for two-component system developer.
As such inorganic magnetic material, metal such as iron, cobalt, nickel, alloy of the above metal with other one, or metallic oxide are well known and utilized. When toner is prepared by adding such inorganic magnetic material, the dusting of toner can not be effectively avoided because it is difficult to disperse and incorporate such inorganic magnetic material in binding resin.
Moreover, a spherical toner has such a trouble that it is liable to fly although charged to a high level, because of its small contact with charge giving members such as a carrier or a sleeve).
When color toner is prepared by adding such inorganic magnetic material, it is difficult to effectively enjoy the color of the pigment contained in toner. Especially, when used as transparent color toner, the use of inorganic magnetic powder is accompanied with significant adverse effect.
That is, inorganic magnetic powder itself does not have transparency. Further, it is difficult to uniformly disperse and incorporate inorganic magnetic powder in binding resin, resulting in muddiness as well as marked deterioration of transparency.
Further, toner containing inorganic magnetic powder has a drawback in that it is poorly fixed on a transfer material and the image possesses less smoothness and low density because there remains magnetic material which may not be fused in the fixing step. When the amount of inorganic magnetic powder contained therein is more, such drawback will become more obvious.
Moreover, although such inorganic magnetic materials have such advantages that they are excellent in magnetic properties and stable, they are basically heavy, hard and inferior in processability.